The present invention relates to electric meters and, more particularly, to a light load adjustment for electric meters.
Conventional electric meters employ an aluminum disk driven as a rotor of a small induction motor by an electric field at a speed which is proportional to the electric power being consumed by a load. Geared dials or cyclometer discs integrate the disk motion to indicate the total energy consumed, conventionally measured in kilowatt hours (one kilowatt hour equals one thousand watts of power consumption for one hour).
The speed of the rotor disk is controlled by the competing torques of a rotating electric field and of eddy current losses induced in the disk by rotation within the influence of a permanent magnet. The rotating electric field is produced by one or more voltage coils on one side of the disk and one or more current coils on the other side of the disk. The voltage coils conventionally include a large number of turns of fine insulated wire wound on the center leg of an E-shaped laminated core. The current coils conventionally include a small number of (two or more) turns of heavy conductor on a U-shaped laminated core. The voltage and current coils are arranged to produce a torque in the disk which is equal to the product of the line voltage and the load current; that is, the load power.
In order to calibrate the disk speed, adjustments are provided for full load and light load among others not of interest here. The full load speed of the disk at rated voltage and a test current is factory adjusted by increasing or reducing the magnetic flux of the permanent magnet until the desired relationship between load power and disk speed is achieved. A manual full load vernier adjustment permits shunting of a portion of the magnetic flux of the permanent magnet to provide final control of disk speed at full load.
Even when the disk is adjusted for precise speed at full load, core non-linearities, friction and mechanical or magnetic dissymmetries may produce errors in the relationship between load and disk speed at light loads of, for example, about 10 percent of full load. A light load adjustment is provided to vary the torque applied to the disk at light load either in the direction of motion or against the direction of motion until the desired power and speed relationship is attained. The light load adjustment may conventionally include a light load plate of conductive or magnetic material having a rectangular window punched therethrough or it may include a bar having a pair of tabs of magnetic material interposed between the outer legs of the core of the voltage coil and the disk. When the window or the tabs are symmetrical to the pole of the voltage coil, they contribute zero torque to the disk. The plate or tabs may be displaced parallel to the plane of the disk so that they align with, or shade, one of the outer legs of the E-shaped core of the voltage coil more than they shade the other outer leg. This generates a dissymmetry in the magnetic flux and the resultant torque, produced by the voltage coil which has a magnitude related to the difference in shading of the two outer legs and a direction related to the direction in which the loop or tabs are adjusted.
The light load adjustment is conventionally controlled by rotating a screw which thereby provides a limited translation of the light load plate or tabs. Such a light load adjustment is disclosed, for example, in U.S. Pat. Nos. 4,423,375; 4,424,484 and 4,424,485. A rigid convention has been adopted for the direction of rotation of controls used for adjusting disk speed. According to this convention, turning a control in the clockwise direction slows the disk rotation and turning a control in the counterclockwise direction speeds up the disk rotation.
Some electric meters employ two or three voltage coils in order to combine the power usage data from two or more phases of the load power into rotation of a single disk. In one such arrangement, two voltage coils are arranged 180 degrees apart about the circumference of the disk. Each of the voltage coils may be provided with its own light load adjustment for improved precision under assymmetric loading of the phases. It is convenient to position the light load adjustment screws for both voltage coils facing toward the front of the meter so that a worker has easy access to them from a single location. However, their locations spaced 180 degrees apart about the circumference of the disk requires that the displacements of the light load adjustments must be in opposite directions with respect to the worker in order to obtain the same change in disk speed. This causes a problem in meeting the standard that the direction of rotation of the adjustment screws must produce the same direction of change in disk speed.
Similar problems exist when, for example, three voltage coils are spaced 120 degrees apart about the disk.
One way that such adjustment problems might be solved is to provide two different adjustment linkages, one with a right-hand threaded screw and the other with a left-hand threaded screw. In this arrangement, one of the adjustment linkages may be used on one of the voltage transformers and the other adjustment linkage may be used on the other voltage transformer. This technique suffers the cost disadvantage that two different linkages, and perhaps even two different voltage transformers assembled with their respective linkages, must be manufactured and stocked.